1. Field of the Invention
This invention relates to elastomers and to coating compositions prepared therefrom and is more particularly concerned with polyurethane elastomers, with methods for their preparation and with methods for the preparation of coating compositions therefrom.
2. Description of the Prior Art
Polyurethane elastomers which are soluble in organic solvents, and are accordingly useful in the preparation of coating compositions, are well-known in the art. It has not been found possible to prepare a substantially completely cured solid polyurethane elastomer which has good physical properties, such as tensile strength, elasticity, and the like, and which is also sufficiently soluble in organic solvents to be useful in the preparation of coating compositions to be employed as paints, sealants, adhesives and the like.
Thus, in order to achieve a solvent soluble polyurethane elastomer it was generally necessary to employ low NCO/OH indexes, i.e. 0.95:1 or even lower. The physical properties of such products are poor and are reflected in the behaviour of coating compositions prepared therefrom.
Accordingly, many of the polyurethane elastomer coating compositions which are presently available comprise solutions of isocyanate-terminated prepolymers which are cured at the time of application, either by addition to an extender immediately prior to application, or by exposure to atmospheric moisture after application. It is only in the curing stage that the "hard" segments of the polyurethane elastomer are introduced. The introduction of these segments at any earlier stage reduces the solvent solubility of the product to unacceptable levels.
In preparing such isocyanate-terminated polyurethane prepolymers for use in coating compositions, monohydric alcohols and like chain terminators are frequently used to control viscosity, hardness and like properties; see, for example, U.S. Pat. Nos. 3,384,623; 3,350,361; 3,823,111; 3,761,439; and 3,483,167. The use of chain terminators in the preparation of polyurethane elastomers in the presence of organic solvents, i.e. the preparation of coating compositions without isolation of the solid polymer, has been described; see, for example, U.S. Pat. No. 3,425,973. However, it is found that such techniques do not give polyurethanes of reproducibly consistent properties and, accordingly, such techniques are not useful commercially where consistency of product properties from batch to batch is essential.
The use of chain terminators of the above types, including relatively high molecular weight monohydric alcohols, in the preparation of polyurethane elastomers to control hardness therein is also known in the art; see, for example, U.S. Pat. Nos. 3,272,098 and 3,875,086. However, the products in question are not solvent-soluble and are not shown to be capable of conversion to solvent soluble materials and hence to be useful in solvent containing coating compositions.
Very recently a process has been described (U.S. Pat. No. 3,887,525) for the preparation of solvent-soluble polyurethane elastomers which process involves heating, at 180.degree. to 220.degree. C, a polyurethane derived by reacting a diol extender with an isocyanate-terminated prepolymer obtained from a diisocyanate and a polypropylene glycol. In a preferred embodiment of the process the heating step is coupled with mechanical working, e.g. extrusion, of the polyurethane. The process is said to render the polyurethane soluble in "powerful" solvents by which are meant dimethylformamide, dimethylacetamide and dimethylsulfoxide none of which is conventionally employed (because of toxicity considerations) in a coating composition.
It has now been found that substantially completely cured polyurethane elastomers can be prepared which, although not themselves soluble at ambient temperatures in polar organic solvents such as those commonly employed in coating compositions, can be readily converted to a form in which they will pass into solution in said solvents. Further, the physical properties of the polyurethane elastomers in question do not deteriorate significantly as a result of the processing and the resulting coating compositions possess highly advantageous properties.